Discontinuous transmission CDMA system

ABSTRACT

A method of transmitting from a first node to a second node, said method comprising the steps of transmitting in a first mode at least one data frame from said first node to said second node; and determining if there is no data to be transmitted and if there is no data to be transmitted transmitting at least one empty frame in the first mode; and switching to a second mode in which no transmission is made in the absence of data to be transmitted at least some of the time.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Application Ser. No. 09/959,034filed Jan. 8, 2002, which issued on Jul. 31, 2012 as U.S. Pat. No.8,233,442, which was the National Stage of International Application No.PCT/EP00/03453 filed Apr. 17, 2000, which claims the benefit of UnitedKingdom Application No. 9908805.6 filed Apr. 16, 1999 and United KingdomApplication No. 9909689.3 filed Apr. 27, 1999. Each of the abovereferenced applications is hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to a method of transmitting and inparticular but not exclusively to a method of transmitting in a wirelesstelecommunications network.

BACKGROUND OF THE INVENTION

The use of code division multiple access (CDMA) is being proposed forthe next generation of cellular telecommunication networks.Additionally, code division multiple access is also being used in theIS-95 Standard in the USA. CDMA is a direct sequence spread spectrumtechnique. In a wireless cellular network using CDMA, the mobileterminals in one cell associated with a first base station will use thesame frequency as mobile stations in an adjacent cell associated with asecond base station. The different mobile stations can be distinguishedby the respective base stations as each mobile station will be using adifferent spreading code.

In U.S. Pat. No. 5,101,501 a CDMA system is described which uses “soft”handoff. With soft handoff, a mobile station is capable of communicatingwith more than one base station at the same time. This will typicallyoccur when the mobile station is close to the boundary defined betweentwo cells. The signals sent by the mobile stations will be received andprocessed by both of the base stations. Likewise, the mobile stationwill receive the same signal from the two base stations. The signalsfrom the two base stations may be combined. The combined signal mayprovide better quality than any of the individual signals received bythe mobile station.

Thus, one mobile station can listen to two or more base stationssimultaneously. It is therefore important that the information receivedby the mobile stations from the different base station be identical. Ifdifferent information is received from the different base stations, themobile station will have no way of determining which is the correctinformation and which information can be ignored. Additionally it willnot be possible to combine the signals from the different base stationsto provide a meaningful combined signal.

Generally, different information will be received by the mobile stationbecause an error in transmission has occurred and one of the expectedtransmissions from one or more of the base stations is not transmittedby the base station. The base station may not receive any information tobe transmitted to the mobile station for two reasons. Firstly there hasbeen an error in the information received by the base station. Secondly,there is in fact no information to be received. The lack of informationmay be due to, for example a pause in conversation or because thetransmission is a discontinuous transmission. It is therefore necessaryto determine whether the lack of received information is intentional orwhether it is due to an error.

It has been suggested by the inventor that empty frames of data can besent to the base station in two different ways. These two ways do notconstitute prior art. Firstly, the transmission is suspended. In otherwords, the base station will be sent nothing. This has the advantagethat the processing resources at the base station are saved as well asin other network elements. The load at the interface between the basestation and the network controller, which is sending the information tothe base station, is reduced. However this method has the disadvantagethat it is not possible to distinguish between a transmission error andthe fact that there is no data. This leads to two problems.

Firstly, failures in the interface between the base station and thenetwork controller are not detected by the base station in the case ofinformation being sent from the network controller to the base stationor by the network controller in the case of information being sent fromthe base station to the network controller.

Secondly, if a data frame is not received, due to an error, by a basestation, that data frame cannot be transmitted by the base station tothe mobile station. If the mobile station is in soft handoff, that iscommunicating with more than one base station at a time, the mobilestation will receive different data streams from the base stations dueto the error. This may lead to errors in the mobile station when itattempts to combine the different data streams.

The second method for dealing with empty frames of data is for the basestation to be sent empty frames of data. This makes it possible todistinguish between errors and the fact that there is no information.However, this method does have the disadvantage that additionalprocessing capacity is required at the base station and other networkelements. Additionally, the load at the interface between the basestation and the network controller is increased.

Similar problems can also occur in the uplink directions.

SUMMARY OF THE INVENTION

It is an aim of embodiments of the present invention to provide a methodof dealing with the absence of information or data which addresses thedisadvantages of the known methods.

According to one aspect of the present invention, there is provided amethod of transmitting from a first node to a second node, the methodcomprising the steps of transmitting in a first mode at least one dataframe from the first node to the second node; and determining if thereis no data to be transmitted and if there is no data to be transmittedtransmitting at least one empty frame in the first mode; and switchingto a second mode in which no transmission is made in the absence of datato be transmitted at least some of the time.

The first and second nodes may be any suitable elements in atelecommunications network.

According to a second aspect of the present invention there is provideda method of receiving transmissions from a first node at a second node,the transmission has a first mode in which if there is no data to betransmitted, empty frames are transmitted and a second mode in which ifthere is no data to be transmitted nothing is transmitted, the methodcomprising the steps of: receiving transmissions from the first node,wherein if the transmissions have a first mode and a frame is notreceived an error is determined to have occurred.

According to a third aspect of the present invention there is provided afirst node for transmitting data to a second node, wherein the firstnode comprises transmitting means for transmitting data to the secondnode, the transmitting means have a first mode of operation in whichdata is transmitted in data frames and if there is no data to betransmitted empty frames are transmitted and a second mode of operationin which if there is no data no frames are transmitted.

According to a fourth aspect of the present invention, there is provideda second node for receiving transmissions from a first node at a secondnode, the transmissions have a first mode in which if there is no datato be transmitted, empty frames are transmitted and a second mode inwhich if there is no data to be transmitted nothing is transmitted, thesecond node comprising means for receiving transmissions from the firstnode, and determining means for determining that if the transmissionshave a first mode and a frame is not received, an error has occurred.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and as to how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying drawings in which:

FIG. 1 shows a schematic diagram of part of a cellulartelecommunications network incorporating base transceiver stations andmobile stations;

FIG. 2 shows part of the network of FIG. 1 in more detail; and

FIG. 3 illustrates the normal and silent modes used by the base stationsof FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Reference will first be made to FIG. 1 in which three cells 2 of acellular telecommunications network are shown. Each cell 2 is served bya respective base transceiver station (BTS) 4. Each base transceiverstation 4 is arranged to transmit signals to and receive signals fromthe mobile stations 6 located in the cell associated with the given basetransceiver station 4. Likewise, each mobile station 6 is able totransmit signals to and receive signals from the respective basetransceiver station 4.

The cellular telecommunications network shown in FIG. 1 uses a codedivision multiple access technique. Accordingly, at least some of themobile stations will be in communication with more than one base stationat the same time. This, however, will be described in more detailhereinafter.

Reference is now made to FIG. 2. As can be seen from this figure, thebase stations are connected to a SRNC (serving radio network controller)20. Data blocks are transmitted between the SRNC 20 and the basestations 28 and 30 using a frame protocol FP. A data frame contains datawhich is to be transmitted in one interleaving period and a transportformat indicator TFI. The interleaving period is the period over whichdata is interleaved. The transport format indicator TFI providesinformation as to the format of the data frame such as the size of thedata block, the interleaving period and the like.

Each UE (user equipment for example mobile stations or the like) has anindependent transport connection which means that each frame containsthe data and current transport format indicator TFI of one bearer only.Typically a call will have more than one bearer, for example asignalling bearer and a data or voice bearer. For downlinkcommunications (from the SRNC 20 to the base station direction), all thedata blocks of users having the same user equipment are multiplexed ontoa single radio link. The manner in which the multiplexing is done willdepend on the transport format indicator TFI of each frame whichindicates the amount of data in each frame for the bearers. A transportformat combination information TFCI field of the radio frame provided ina dedicated physical control channel DPCCH will indicate the manner inwhich the multiplexing is carried out. The transport format combinationinformation TFCI field will indicate the transport format indicator TFIfor each of the bearers whose data is multiplexed in the radio frame orframes in the dedicated physical data channel DPDCH. In the case of softhandoff where a mobile station is in communication with more than onemobile station at a time, the dedicated physical data channels DPDCHfrom each of the base stations communicating with the mobile station inquestion will be the same as will the rate information RI fields in allthe radio links.

The format described hereinbefore is part of the currently proposed UMTS(universal mobile telephone service) standard. It should be appreciatedthat any other standard can alternatively be used.

In the following, it is assumed for simplicity that there is only onebearer per transport channel and the SRNC does not carry out anymultiplexing. The transport format information can have a particularvalue which indicates that the frame does not contain any data. Forsimplicity it is assumed that the particular value is zero although theparticular value can be any other value.

The SRNC 20 shown in FIG. 2 has first and second medium accesscontrollers MAC 24 which receive the data from respective bearers. Forsimplicity, the medium access controllers 24 are each shown as receivinginformation from one bearer only. The medium access controllers canreceive information from more than one bearer and carry out themultiplexing function described hereinbefore.

The medium access controller 24 receives an input from a first bearerBearer 1 and a second bearer, Bearer 2 and outputs a first transportchannel 1 and a second transport channel 2. The input from Bearer 1 mapsto the first transport channel 1 and the input from Bearer 2 maps to thesecond transport channel 2. The transport channels are input torespective macrodiversity combiners (MDC) 26. The macrodiversitycombiners 26 output the transport channels to each base station which isin communication with the mobile station for which the transport channelis intended. In the example shown in FIG. 2, the first transport channel1 is output to first and second base stations 28 and 30 as is the secondtransport channel 2. Thus the first and second base stations are eacharranged to transmit the same channels to a single mobile station.

The mode of operation for transmission of information between the SRNC20 and base stations will now be described with reference to FIG. 3. Twomodes are defined. The first mode is the normal mode and the second modeis the silent mode. The data which is to be sent to the mobile stationis interspersed with periods where there is no data. This may occurduring pauses in a conversation between two parties. Alternatively, inthe case of non speech data, the data may be discontinuous.

In the normal mode A of operation, on the transmit side, the SRNC 20transmits a frame F for every interleaving period. As shown in FIG. 3,frames F1 and F2 contain data. If no data is present, frames F3, F4 andF5 which are empty frames E are sent. After N consecutively receivedempty frames E, the frame protocol connection is switched to the silentmode S. N can have any suitable value such as 1, 2 or 3 or any othersuitable value.

In the silent mode S, on the transmit side, the SRNC 20 is preventedfrom transmitting any frame if there is no data in the frame. The SRNCmay transmit K empty frames F6 at the beginning of the silent mode S toensure that the base station has switched to the silent mode; also if upto K of the N empty frames previously sent have been lost during thetransmission. After M consecutive frames F7 and F8 including data aretransmitted, the frame protocol connection is switched to the normalmode A again. K may be 0 or 1 or any other suitable number. M can be 1,2, 3 or any other suitable number.

In the normal mode A, on the receive side, if a frame is not received inan interleaving period, the base station generates an interface failurealarm and decides whether to transmit the radio frame without the dataor to not transmit the frame at all . After the base station hasreceived N empty frames, the frame protocol connection is switched tothe silent mode S.

In the silent mode S, on the receive side, if a frame is not received inone interleaving period, the base station considers the bearer to be inan inactive state and sets the transport format indicator TFI to zero.After M consecutive data frames with data have been received, the framecontrol layer (FCL) connection is switched to the normal mode A.

The parameters M, N and K may be:

-   1. predetermined, for example fixed in a standard used by the    network;-   2. lub/lur (lub is the interface between the SRNC and the base    station while lur is the interface between two radio network    controllers) interface protocol configurable parameters, that is the    parameters can be set at each interface as required; and-   3. signalled from the SRNC to base stations every time a new branch    or bearer is set. The parameters may be reconfigurable as required.

Control frames which do not contain data but instead contain controlinformation such as power control and synchronisation information aresent in both the normal mode and the silent mode. Control frames may notbe present in either or both of these modes.

Reference will now be made to a second embodiment of the presentinvention which uses explicit signalling in order to switch from thenormal mode to the silent mode. In this second embodiment, the basestation will not automatically switch to the silent mode when apredetermined number of consecutive empty frames have been received.However, the transmission of a predetermined number of empty frames whenin the normal mode can be used as an indication that the silent modeshould be used. Of course, any other suitable measure can be used todetermine that a switch should be made between the normal and the silentmode.

Once a determination has been made that the base station should stopusing the normal mode and start using the silent mode, the radio networkcontroller will include in the next frame sent to the base station amode bit indicative of this determination. This mode bit may have agiven value for the silent mode and the other value for the normal mode.Alternatively, one value of the bit indicates that the mode shouldchange either from the normal to the silent mode or from the silent modeto the normal mode. The other value of the bit would indicate that themode is not to be changed.

When the base station receives the frame transmitted by the radionetwork controller, it checks the mode bit. If this bit indicates thatthe mode is to change from the normal mode to the silent mode, the basestation changes its mode and sends an acknowledgement confirming thatthe base station has understood that the mode is changed. Once the radionetwork controller has received the acknowledgement from the basestation, the radio network controller will use the silent mode.

Changes from the silent mode to the normal mode can be achieved in thesame way as in the first embodiment. However, it is also possible toalter the mode from the silent to normal mode in the following. When itis determined that the base station is to receive using the normal mode,the radio network controller will alter the mode bit in the next framesent to the base station, to indicate that the mode should change to thenormal mode. If a data frame is being sent to the base station, the modebit will be altered in that frame. If no data frame is being sent to thebase station due to an absence of data, the radio network controllerwill send an empty frame with the mode bit set accordingly.

The base station will send an acknowledgement indicating that it hasreceived the frame and has noted that the mode is changed to the normalmode. When the radio network controller receives the acknowledgement,the radio network controller will then use the normal mode.

It is possible to use this second method to notify the base station bysending in advance information that the mode will change after apredetermined number of frames or the like.

The method used by the first embodiment has the advantage that the framestructure does not need to be altered. A second advantage of the firstmethod is that two way communication does not have to be set up if oneparty, for example, the radio network controller is sending data to thebase station and the base station is not sending any data to the radionetwork controller. The second method is advantageous in that bothparties are certain as to which mode of operation is being used.

The acknowledgement sent by the base station to the radio networkcontroller may use a mode bit in the frame, as described hereinbefore inrelation to the frames sent from the radio network controller to thebase station. Thus the acknowledgement may comprise the mode bit in theframe sent by the base station to the radio network changing to reflectthe new mode. Either of the options for the mode bit describedhereinbefore may be used.

It should be appreciated that although the transmissions from the SRNCto the base station are described as using the normal and silent mode ofoperation, this mode of operation can alternatively or additionally beused for transmissions from the base station to the SRNC. The values ofthe parameters used in the downlink and uplink directions may differ.Embodiments of the present invention may be applied to thecommunications between any two nodes of a telecommunications network,wired or wireless.

It should be appreciated that although embodiments of the presentinvention are particularly advantageous when used in a soft handoff orsimilar context, that is when a mobile station is in communication withmore than one base station at the same time, embodiments of the presentinvention can also advantageously be used where the mobile station is incommunication with one base station at a time. In an alternativeembodiment of the present invention, the normal and silent mode ofoperation may only be used when the mobile station is in communicationwith more than one base station at a time. A different mode of operationmay be used when the mobile station is in communication with only onebase station. The different mode of operation may be one whichincorporates one of the previously known methods for dealing with emptyframes.

Whilst preferred embodiments have been described in the context of acode division multiple access system, embodiments of the presentinvention can be used with any other spread spectrum technique or anyother suitable access technique such as time division multiple access,frequency division multiple and space division multiple access as wellas hybrids thereof.

The invention claimed is:
 1. A method comprising: transmitting in afirst mode at least one data frame from a first node to a second node;and determining if there is no data to be transmitted from said firstnode to said second node and if there is no data to be transmitted,transmitting from said first node to said second node at least one emptyframe in the first mode; switching to a second mode, wherein in saidsecond mode no transmission of any frame is made from said first node tosaid second node in the absence of data to be transmitted from saidfirst node to said second node; and determining, in said second mode, ifdata is to be transmitted from said first node to said second node andif there is data to be transmitted, transmitting at least one data framefrom said first node to said second node.
 2. A method as claimed inclaim 1, comprising sending information from said first node to saidsecond node advising of a change from said first mode to said secondmode.
 3. A method as claimed in claim 2, comprising receiving at saidfirst node confirmation of receipt of said information and then usingsaid second mode.
 4. A method as claimed in claim 3, comprisingincluding said information in a frame transmitted from said first nodeto said second node.
 5. A method as claimed in claim 3, comprisingsending said information in advance of a change of mode.
 6. A method asclaimed in claim 1, comprising switching automatically to said secondmode if a predetermined number of empty frames are transmittedconsecutively.
 7. A method as claimed in claim 1, comprising switchingfrom said second mode to said first mode if a predetermined number ofdata frames are transmitted.
 8. A method as claimed in claim 7,comprising switching automatically to said first mode if thepredetermined number of data frames is received in said second mode. 9.A method as claimed in claim 7, comprising, when said first node isusing said second mode and mode is to be switched to said first mode,transmitting from said first node information that said first mode is tobe switched to said second mode.
 10. A method as claimed in claim 9,comprising switching to said first mode when receiving at said firstnode from said second node confirmation receipt of said information thatsaid mode is to be switched from said second mode to said first mode.11. A method as claimed in claim 7, wherein said predetermined number offrames are consecutive frames.
 12. A method as claimed in claim 1,comprising transmitting from said first node the same data to aplurality of second nodes, for transmission by said plurality of secondnodes to a third node.
 13. A method as claimed in claim 1, wherein saidfirst node is a base station or a radio network controller in a wirelesscellular telecommunications network.
 14. A method as claimed in claim 1,wherein said second node is a base station or a radio network controllerin a wireless cellular telecommunications network.
 15. A methodcomprising, transmitting in a first mode at least one data frame from afirst node to a second node; and determining if there is no data to betransmitted and if there is no data to be transmitted, transmitting atleast one empty frame in said first mode; switching to a second mode,wherein in said second mode no transmission is made in the absence ofdata to be transmitted, wherein if no data is to be transmitted at thebeginning of said second mode, first transmitting a predetermined numberof empty frames and then making no transmission of any frame in theabsence of data to be transmitted; and determining, in said second mode,if data is to be transmitted from said first to said second node and ifthere is data to be transmitted, transmitting at least one data framefrom said first node to said second node.
 16. An apparatus comprising:transmitting means for transmitting data to a second node, saidtransmitting means have a first mode of operation in which data istransmitted in data frames and if there is no data to be transmitted,empty frames are transmitted, and a second mode of operation in which ifthere is no data to be transmitted, no frames are transmitted, whereinsaid transmitting means is configured to send information to said secondnode advising of a change from said first mode to said second mode,wherein said transmitting means determines, in said second mode, if datais to transmitted to said second node and if there is data to betransmitted, transmits at least one data frame to said second node. 17.A method comprising: transmitting in a first mode at least one dataframe from a first node to a second node; and determining if there is nodata to be transmitted from said first node to said second node and ifthere is no data to be transmitted, transmitting from said first node tosaid second node at least one empty frame in said first mode, whereinsaid at least one empty frame includes transport format informationindicating that the frame does not contain any data; switching to asecond mode, wherein in said second mode no transmission of any frame ismade from said first node to said second node in the absence of data tobe transmitted from said first node to said second node; and determiningif data is to be transmitted from said first to said second node and ifthere is data to be transmitted, transmitting at least one data framefrom said first node to said second node.
 18. A method as claimed inclaim 17, comprising transmitting data in data frames from said firstnode to said second node whilst in said second mode when there is datato be transmitted from said first node.
 19. A method as claimed in claim17, comprising switching from said second mode to said first mode if apredetermined number of data frames are transmitted.
 20. A method asclaimed in claim 17, comprising when switching from the second mode tothe first mode, transmitting from said first node to said second nodeinformation advising of a switch to said first mode.
 21. A method asclaimed in claim 17, comprising if no data is to be transmitted at thebeginning of said second mode, first transmitting a predetermined numberof empty frames and then making no transmission of any frame in theabsence of data to be transmitted.
 22. A method as claimed in claim 17,comprising: transmitting the same data to a plurality of second nodes,for transmission from said plurality of second nodes to a third node.23. An apparatus comprising: a transmitter configured to transmit datato a second node, said transmitter having a first mode of operation inwhich the transmitter transmits data in data frames and if there is nodata to be transmitted, transmits empty frames, a second mode ofoperation in which the transmitter transmits no frames if there is nodata to be transmitted; and said transmitter is further configured todetermine, in said second mode, if data is to transmitted to said secondnode and if there is data to be transmitted, transmits at least one dataframe to said second node.
 24. The apparatus as claimed in claim 23,wherein said transmitter is further configured to send information tosaid second node advising of a change from said first node to saidsecond node.
 25. The apparatus as claimed in claim 23, wherein saidtransmitter is further configured to determine there is no data totransmit if said transport format information of a frame comprises avalue indicating that said frame does not contain any data.
 26. Theapparatus as claimed in claim 23 is a base station or a radio networkcontroller in a wireless cellular telecommunications network.
 27. Theapparatus as claimed in claim 23, wherein said transmitter is furtherconfigured to switch automatically from said first mode to said secondmode if a predetermined number of empty frames are transmittedconsecutively.
 28. The apparatus as claimed in claim 23, where saidtransmitter is further configured to switch from said second mode tosaid first mode if a predetermined number of data frames aretransmitted.
 29. A method comprising: operating in a first mode in whicha data frame is transmitted from a first node to a second node for eachtime period for which there is data to be transmitted from said firstnode to said second node, and an empty frame is transmitted from saidfirst node to said second node for each time period for which there isno data to be transmitted; and switching to a second mode in which noframe is transmitted from said first node to said second node for eachtime period for which there is no data to be transmitted, anddetermining if data is to transmitted from said first node to saidsecond node and if there is data to be transmitted, transmitting the atleast one data frame from said first node to said second node.
 30. Anapparatus comprising: a transmitter configured to transmit data from afirst node to a second node in data frames, said transmitter having afirst mode of operation in which the transmitter transmits an emptyframe from said first node to said second node for each time period forwhich there is no data to be transmitted; a second mode of operation inwhich the transmitter transmits no frame from said first node to saidsecond node for each time period for which there is no data to betransmitted, wherein said apparatus is further configured to sendinformation to said second node advising of a change from said firstmode to said second mode; and determining if data is to transmitted fromsaid first node to said second node and if there is data to betransmitted, transmitting the at least one data frame from said firstnode to said second node.